Method of making crystals of rare earth chalocogenides



Feb. 27, 1968 c. F. GUERCI 3,370,924

METHOD OF MAKING CRYSTALS OF RARE EARTH CHALCOGENIDES Filed Aug. 30, 1965 FIG. 1

He 0m SEALED CRUCIBLE FIG. 2

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1111 T 2 H8 COyDENSED Eu METAL Q PURE EuOCRYSTAL 22 /M|XTURE 0F Eu+Eu 0 INVENTORS CARL F. GUERCI MERRILL W. SHAFER BY M Flea A ORNEY United States Patent Ohfice 3,370,924 Patented Feb. 27, 1968 3,370,924 METHOD OF MAKING CRYSTALS OF RARE EARTH CHALCOGENIDE Carl F. Guerci, Mount Kisco, and Merrill W. Shaffer, Yorktown Heights, N.Y., assignors to International Business Machines Corporation, Armonlr, N.Y., a corporation of New York Filed Aug. 30, 1965, Ser. No. 483,712 Claims. (Cl. 23301) ABSTRACT OF THE DISCLOSURE A rare earth chalcogenide selected from the group consisting of europiurn', ytterbium and samarium is placed together with the elemental metal of the chalcogenide compound in a crucible selected from the group consisting of tantalum, tungsten, molybdenum and rheniurn, sealed and heated to solution temperature and subsequently cooled.

This invention relates to a process for manutacturing pure, single crystals of divalent rare earth chalcogenides.

The chalcogenides are compounds made up of the elements in Group VI of the Periodic Table and would consist of compounds that contain 0, S, Se, and Te. The rare earth chalcogenides would be a compound that comprises a rare earth and an element from Group Vl. Representative rare earth chalcogenides are Euo, EuS, etc., or YbO, YbS, etc., or SmO, SmSe, etc. Some of these compounds, for example But), have extremely high magnetic moments and extremely high Verdet constants, the latter constant being a measure of a materials ability to rotate the plane of polarization of light entering the material when a magnetic field is applied to such material. EuO also has low magnetic anisotropy, thus requiring relatively low magnetic fields to change its radiation transmitting characteristics.

In previous attempts to grow single crystals of rare earth ohalcogenides, difliculty was encountered because the rare earths would react with the container in which the rare earths were heated and upon cooling, the metal of the crucible container would precipitate out into the rare earth crystal as an impurity. Highly stressed crystals were also common when a melt was cooled under normal crystal growing conditions. Further, if pure EuO were placed in a crucible such as tantalum, tungsten, molybdenum and the like and heated to 1900 C., it was found, upon subsequent cooling to room temperature, that the resulting europium oxide crystal was poor. Even in cases where no crucible material was dissolved in the europium oxide, the crucible either changed the ratio of europium metal to oxygen, or, because of compositional diiferences between the liquid and solid phase, the resulting crystal contained many inclusions of another phase. The 1:1 ratio of europiurnto oxygen was disturbed so that a phase other than EuO existed in the end product. On occasion, Eu O was added to the europium oxide in the crucible but still non-stoichiometric EuO crystals were obtained.

The present invention has devised a process for growing single crystals of divalent rare earth chalcogenides that are pure (no inclusions), have high crystallinity, and retain their high magnetic moments. When highly polished, such crystals are very transparent to a selected range of the electromagnetic spectrum.

Consequently, it is an object of this invention to obtain improved single crystals of divalent rare earth chalcogenides.

It is yet another object to obtain larger single crystals of such rare earth chalcogenides than have hereto been obtainable.

It is still another object to obtain large single crystals of divalent rare earth chalcogenides that are of exceptional purity.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

in the drawings:

FIG. 1 is a showing of a crucible containing the unmeited charge of rare earth oxide and rare earth metal in which the new, large single crystal is to be grown.

FIG. 2 is a schematic showing of a furnace containing the crucible in which the new crystal is to be grown.

FIG. 3 is a cross-section of the novel crystal material after slow cooling from a molten state.

A description of the new process for growing large single crystals of divalent rare earth chalcogenides can better be understood by beginning with a discussion of FIG. 1 of applicants drawings. In describing the invention discussed hereinabove, applicants have taken europium metal and europium oxide (E110) as their example for obtaining a new rare earth chalcogeuide crystal. It is to be understood that, with slight variations, the process to be described can be carried out for obtaining large, single crystals of compounds other than europium oxide and europium'.

Eu metal and EuO are placed in a crucible 2, the latter being made of a material that will not react with the Eu metal and EuO when such materials are in their respective molten or vapor states. The EuO and Eu are originally in powder form and are compressed into pellets 3 and 5', etc., before being placed into the crucible 2. Tungsten, rhenium, and molybdenum are recommended crucible materials. The compositional range for the two materials is -25 mol percent of EuO and 2575 mol percent of Eu metal. The container 2, with the Eu metal and EuO placed therein, is inserted in a suitable welding chamber containing a protective atmosphere such as argon or helium.

A cap or cover 4 of tungsten, rhenium, molybdenum or the like is arc-welded to the crucible or container 2 while in such welding chamber to make a gas tight closure for the latter. After the cap 4 has been welded to container 2, the closed crucible is placed in a suitable furnace which is shown schematically in FIG. 2. Such furnace includes a silica glass support 6 containing Lampblack 7 surrounding a carbon susceptor 8 containing the crucible 2. The crucible 2 is positioned within a second silica glass tube 12 about which is wrapped a coil 14. A source of electrical energy, not shown, supplies high-frequency current to coils 14 so as to induction heat the sealed crucible 2 and its contents.

In the actual process of induction heating of the sealed crucible 2, the top portion (about /2-inc-h of a 6-inch crucible) of such crucible protrudes beyond the top of the carbon susceptor 8. When the induction heating raises the sealed crucible 2 to a temperature of 2000" C. to 1650 C., depending upon the metal Eu to EuO ratio, the protruded portion remains about 200 or more degrees cooler, causing the bottom of the crucible to be region where the molten EuO and En metal reside and the upper portion the region where the vapors of EuO and Eu accumulate. By well-known conventional temperature monitoring techniques, the crucible 2 is maintained at this high temperature (2000 C. to 1650 C.) for 2 3 hours, uniformly lowered to 1800" to l450 C. for a period of l620 hours, and then cooled to room temperature for a period of 6-8 hours.

After cooling, the crucible 2 is cut along its length, and FIG. 3 is a cross-ectional view of the end product of the process described above. At the very bottom of the crucible is a layer 26 of solidified Eu metal. Immediately above layer 2:? is an intermediate zone 22 that contains a mixture of En metal and EuO. Above zone 22. is the pure EuO layer 24, the latter having a coating 26 of condensed Eu metal vapor. Region 2-8 is the evacuated space resulting from the condensation of the Eu metal vapor onto the EuO 24. When the E110 crystal 2% is cut between surfaces A and B of FIG. 3 and polished, the resulting EuO crystal, when subjected to light and X-ray tests, proves to be a crystal having an extremely high degree of crystallinity. Moreover, at low temperatures. the E110 crystal is a ferromagnetic having a high magnetic moment and high Verdet constant. The crystal is large, single phase and mechanically sound.

When this process is practiced using ytterbium or samarium instead of europium, the process is substantially the same but the initial temperature to which the crucible 2 is raised will be 200-400 degrees higher than that employed when europiurn and EuO are used. By practicing the teachings of the present invention, one can obtain new divalent rare earth chalcogenide crystals of extremely high quality. Moreover, such crystals, one centimeter in length or longer, are fairly large. However, such crystals (compounds of ytterbium and sarnarium) are not presently useful as magnetic materials.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein Without departing from the spirit and scope of the invention.

What is claimed is:

1. A method for producing pure single crystals of divalent rare earth chalcogenides comprising the steps of introducing a rare earth compound selected from the group consisting of europium, ytterbium and samari um chalcogenides and the elemental rare earth metal forming said compound into a non-contaminating crucible selected from the group consisting of tantalum, tungsten, molybdenum and rhenium, the said composition of compound and metal being in a ratio sufiicient to form a solution when heated,

heating said crucible at a temperature suflicient to remove most gases, sealing said crucible after such removal of gases,

heating said crucible to an initial solution temperature of 1650 to 2400 C. for a period of 2 to 3 hours,

uniformly lowering the temperature of said crucible to 1800 to 1450 C. for a period of 16 to hours, and then cooling said crucible to room temperature for a period of 6 to 8 hours.

2. A method for producing pure single crystals of divalent rare earth chalcogenides comprising the steps of:

mixing a rare earth compound selected from the group consisting of europium, ytterbium and samariurn chalcogenides and the elemental rare earth metal forming said compound in an open crucible selected from the group consisting of tantalum, tungsten, molybdenum and rhenium, the said composition of compound and metal being in a ratio of -25 mol percent of the rare earth compound and 2575 mol percent of the metal, sealing said crucible so as to make the latter gas tight, maintaining said sealed crucible at an initial solution temperature of 1650" to 2400 C. for a period of 2 to 3hours,

uniformly lowering the temperature of said crucible from said initial temperature to a temperature of 139-3- C. to 1450" C. for a period of 1620 hours,

nd then cooling said crucible to room temperature for a period of 6 to 8 hours. 3. The method of claim 2 wherein the rare earth compound contains an oxide of europium and the metal mixed therewith is europium.

4. The method of claim 2 wherein the rare earth compound is an oxide of ytterbium and the metal mixed therewith is ytterbium.

5. The pound is an oxide of Samarium and the metal mixed therewith is samarium.

6. The method of claim 2 wherein the rare earth compound is a selenide of europium and the metal mixed therewith is europium.

7. The method of claim 2 wherein the rare earth compound is a selenide of ytterbium and the metal mixed therwith is ytterbium.

S. The method of claim 2 wherein the rare earth compound is a selenide of Samarium and the metal mixed therewith is Samarium.

9. A method for producing pure single crystals 0 europium oxide comprising the steps of: introducing europium oxide and europium into a noncontaminating crucible selected from the group consisting of tantalum, tungsten, molybdenum andrhenium, said europium oxide and europium being in a ratio suflicient' to form a solution when heated, wherein the europium oxide comprises 7525 mol percent of the mixture and the europium metal is 25-75 mol percent of the mixture, sealing said crucible so as to make the latter gas tight, heating said crucible at an initial solution temperature of 2000 C.1650 C. for a period of 2 to 3 hours,

uniformly lowering the temperature of said crucible from said initial temperature to a temperature of 1800" C.1450 C. for a period of 16-20 hours, and then cooling said crucible to room temperature for a period of 6 to 8 hours.

19. The method of claim 9 wherein a portion of one end of the crucible is maintained at a lower temperature than the rest of the crucible during said heating and cooling steps.

References Cited UNITED STATES PATENTS 2,735,155 2/1956 Glaser 26466 2,755,747 2/1956 Kasey 23-16 3,133,788 5/1964 Kern 2321 NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner.

method of claim 2 wherein the rare earth com- 3 

